The present invention relates to a diagnosis system for a motor vehicle, and which is capable of diagnosing trouble in a plurality of electronic control units which are mounted on the motor vehicle.
In conventional technology, the electronic control units control various amounts such as an air-fuel ratio by using a microcomputer. Ordinally, the units have a self-diagnosis function for indicating troubles by lighting or turning on and off a self-diagnosis lamp when trouble occurs in sensors or actuators.
However, it is impossible for users to easily understand an extent such as a position or degree of the trouble by only lighting or turning on and off the lamp. Accordingly, service stations of dealers must have diagnosis systems for vehicles with trouble, and which are capable of easily checking data about the positions or degree of the troubles occurring in the electronic control units. Such diagnosis systems are, for example, disclosed in Japanese Patent Laid-open No. 58-12848 (1983).
However, the motor vehicle has a plurality of control units for controlling an engine, a transmission, a brake, and a steering mechanism. In the case of diagnosing troubles in the control units, a connector of the diagnosis system must be connected one by one to connectors of the control units, and the diagnosis operation is complicated. Furthermore, in the case of diagnosing troubles influencing a plurality of control units, the diagnosis operation is difficult and it takes much time in order to find the trouble. In order to solve problems, the applicant of the present invention has already proposed a diagnosis system comprising a diagnosis unit for finding troubles in the vehicle, a communication bus consisting of a transmitting line and a receiving line, a plurality of control units connected to the communication bus, and a connector provided at an end of the bus for connecting the control units to the diagnosis unit. And the diagnosis system performs a communication between the control units and the diagnosis unit through one connector.
However, the control units have different processing abilities corresponding to controlled systems, respectively, and thereby having different abilities for processing the communication. Accordingly, all control units do not necessarily have the same communication system, so that each unit has a start-stop or a clock synchronous communication system or the like.
Therefore, the control units having the same communication system are capable of being connected to the diagnosis system by one connector. But, it is problem that the diagnosis system needs some connectors corresponding to every communication system when one vehicle has a plurality of the control units respectively having different communication systems.